Abstract

The principal taxol biotransformation reaction of humans and of human hepatic in vitro preparations is 6α-hydroxylation of the taxane ring, but a separate, minor hydroxylation pathway (metabolite B formation) also exists. Taxol metabolism was studied using membrane fractions from Hep G2 cells infected with recombinant vaccinia viruses that contain complementary DNAs encoding several human cytochrome P450 enzymes. Only P450 2C8 formed detectable 6α-hydroxytaxol. Metabolite B formation was catalyzed by complementary DNA-expressed 3A3 and 3A4, but not by 3A5. Each P450 3A preparation catalyzed felodipine oxidation. The apparent Km and Vmax values for taxol 6α-hydroxylation were 5.4 ± 1.0 µm and 30 ± 1.5 nmol/min/nmol P450, respectively, for complementary DNA-expressed P450 2C8; the values were 4.0 ± 1.0 µm and 0.87 ± 0.06 nmol/min/mg protein, respectively, for human hepatic microsomes. The inhibition of 6α-hydroxytaxol formation by quercetin was competitive with an apparent Ki of 1.3 or 1.1 µm with 2C8 or hepatic microsomes, respectively; retinoic acid was inhibitory, showing an apparent Ki of 27 µm with hepatic microsomes; inhibition by tolbutamide was complex, weak, and unlikely to be clinically relevant. The correlation between hepatic 2C8 protein content and 6α-hydroxytaxol formation was high (r2 = 0.82), while the correlation with 2C9 content was low (r2 = 0.38).

These data show that human biotransformation routes of taxol result from catalysis by specific enzymes of two P450 families and that taxol 6α-hydroxylation is a useful indicator of P450 2C8 activity in human hepatic microsomes.

Footnotes

↵1 The United States Adopted Name Council has assigned the name “paclitaxel” to taxol and TAXOL.